1. Field of the Invention
This application is related to U.S. patent application Ser. No. 324,343, filed on common date herewith and also assigned to Fairchild Camera and Instrument Corporation.
This invention relates to a self-refreshing memory cell suitable for use in an integrated circuit with increased packing density over circuits of the prior art, and to its method of operation.
2. Prior Art
Bistable memory cells are well known. Such cells are disclosed, for example, in U.S. Pat. No. 3,562,721 to Norman, issued Feb. 9, 1971. The Norman cell comprises two cross-coupled bipolar transistors connected in what has now become a standard bistable flip-flop configuration. The collector of one bipolar transistor in the cell is connected to the base of the other transistor and the emitters of the two transistors are grounded. When one transistor turns on, the collector voltage on that transistor drops thereby turning off the other transistor. The collector voltage on the off transistor then rises turning on harder the "on" transistor. The state of the cell is changed by pulsing simultaneously the collector of the cross-coupled transistor and a selected switching transistor. The state of the cell is sensed by determining the voltages on the collectors of the two transistors.
Since the disclosure of this long established cell new bistable cells have appeared, including cells using a charge stored at the interface between two dissimilar dielectrics (see, for example, U.S. Pat. No. 3,641,512 issued Feb. 8, 1972 on an invention of Frohman-Bentchkowsky) and cells using so-called "floating gates" which are conductive gates insulated from the active components of the transistor by dielectric. The charge on the floating gate is often controlled by controlling the potential on an overlying word line in such a manner as to either draw a charge from an underlying source to the floating gate or expell charge from the floating gate to the source. Such devices, often making use of electron tunnelling through a thin dielectric, are described, for example, in an article entitled "Low Power EEPROM Can Be Reprogrammed Fast", published in Electronics, July 31, 1980, by Shelton. The EEPROM, short for "Electrically Erasable Programmable Read Only Memory", has distinct advantages over the prior art memories in that the EEPROM can be erased by programming internal to the chip whereas the standard EPROM is erasable only by UV light from an external source. In addition, the EEPROM lends itself to rapid reprogramming in a simple manner with portable equipment.